Kinda hard unless you can get to her clothing secretly. If it is one with a built in type bra you will need her bra size, if it is a loose top you will need to know what size tops she wears. If there is anyway you could sneak a peek at these clothing items to check the size they always have little tags that will tell you (unless she cuts them out.. then you are just going to have to guess and get a gift receipt in case she needs to exchange it) Just be careful if you don't have the type of relationship where she won't get freaked if she catches you rooting through her drawers or closet ;) (hint when in doubt buy the smaller size, most women get offended if they are given something 3x too big "is that how fat you think I am!?" LOL )
Well if you are buying lingerie I am guessing you have at least some access to her bra. you check the tag if you don't know her bra size. And her is a note for all of you men. When your girlfriend or wife is either out or in the shower, go thru her closet and drawers and write down her sizes and keep them in your wallet. Then you will always know and not have to guess.
Can offer a few common sense suggestions but you might end up in trouble if you get caught going thru her drawers & invading her privacy. Or carrying her underwear around in your pocket, lol. Just tell her you saw a cute T-shirt in the store & might buy it for her for Christmas - if they have it in her size?